1. Field of the Invention
The present invention is an improved cylindrical tank that is known in the petroleum industry as a gun barrel. The present gun barrel is used to separate small amounts of water from large amounts of crude oil. The improvements in the gun barrel include features that create more uniform distribution of inlet fluid within the tank and more uniform collection of outlet fluid in the form of oil from the tank.
2. Description of the Related Art
Oil-water separation became a necessity in Drake's first well in Titusville, Pa. As that well began to water out, a system to separate the water was needed. The so-called “gun barrel” tank was developed to meet that need.
Gun barrel designs took a couple of generations to mature. But when they finally did, they became a standard of the industry that has survived ever since. The gun barrel tank, which is also called a “wash tank” in some circles, became universally applied when the industry needed to separate water and oil. The gun barrel design focuses on separating small amounts of water from large amounts of oil.
In the earliest days, most wells produced enough natural gas to cause the early gun barrel designers to add a liquids-gas separator to their tank designs. Removing the gas before the oil and water entered the gun barrel was observed to be necessary since the gas that evolves from liquids tends to stir or mix the water and oil. Even then, i.e. approximately 150 years ago, it was obvious that mixing is detrimental to the process of separating water and oil. So, a gas separator was added to the gun barrel tank. It was referred to as a “degassing boot”, or simply a “gas boot”. As the industry grew, the gun barrel became well known as the preferred way to pre-condition crude oil so it could be refined.
For the next 150 years, the gun barrel remained essentially unchanged, staying true to its original very simple design. That design used a pipe inserted through the roof of a standard tank and extended a few feet above the roof to form the gas boot. The pipe extended down to a few feet above the tank bottom. A horizontal plate was welded onto or bolted to the bottom of the pipe near its bottom to distribute the inlet oil throughout the bottom of the tank. Because of being lighter in density, the oil would rise through the water and the water in the tank was thought to “wash” the water out of the oil. The oil continued to rise within the tank until it reached an oil layer located above the water. Most folks believed that the oil layer needed to be sufficiently large enough to contain all of the crude oil produced in a period of about 8-24 hours.
As time passed, more and more people accepted the fact that the depth of the oil layer needed to be based on the density difference between the oil and the water. It was found that lighter oil could be dehydrated in fewer than eight hours of calculated oil retention time in a gun barrel, while heavier oil might require 24 hours or more.
Also, some gun barrel manufacturers, bowing to the wishes of their customers, placed the gas boot and down corner outside the tank to minimize the corrosive effects of salt water located within the tank. When a tank was configured this way it was often referred to as a wash tank. When the gas boot was located concentrically on top of the tank, the tank was generally referred to as a gun barrel. Although the tanks look different, the functional results are the same.
This design remained unchallenged for nearly 75 year, and was the standard of the industry until the 1930's when heater treaters were developed to lower crude oil viscosity by applying heat to the mixture. These heater treaters were pressure vessels designed to separate and capture the otherwise vented natural gas stream.
But in fields where gas production was non-existent or was depleted, the gun barrel was, and still is, the system of choice.
Finally, in the 1970's, when the price of crude oil soared from about $5.00/barrel to around $45/barrel, this oilfield standard was challenged by some of the industry's more forward thinking oilfield research and develop (R&D) laboratories. The results from these labs were startling and very disappointing.
Report after report showed that the hydraulic efficiency of the traditional wash tank or gun barrel designs was extremely low, most ranging from 1% to 3%. Bigger tanks had been thought to be better for separating oil from water, but in most cases, test showed that bigger tanks actually performed worse. Oil industry separation designs had always been assumed to work as they should, but these studies proved otherwise.
One problem with previous gun barrel designs is that it had always been presumed that fluids naturally distributed uniformly and flowed through the entire gun barrel tank uniformly. Instead, these same studies proved that the oil and water predictably take the paths of least resistance.
Oil and water actually move through the gun barrel tank in the shortest, narrowest, non-uniform flow paths of least resistance, between the fluid inlet and the fluid outlets. Retention time studies proved that only a very small portion of the storage volume of the gun barrel tank was actually in the flow paths of the water and oil. In some tests the inlet crude was found to actually only stay in the oil phase for a matter of minutes, and the water for even less time. In a few test the water retention time was just a few seconds. When compared with the original supposition that oil retention times were hours long, these tests were quite eye opening, and contrary to the assumptions made about gun barrel performance for over 100 years.
At that point, skepticism about the wisdom of the past began to set in. New designs began to shift away from the assumed oil retention time of the past. Designers began to look for new ways to increase fluid distribution and collection systems to optimize the use of the entire vessel, and overall separation efficiency. Every conceivable idea was tried, and surprisingly, most failed.
From this work, more and more researchers agreed that as fluid flow and distribution were rendered more uniform, the separation results were more and more dramatic.
Eventually more and more researchers agreed that truly uniform distribution of incoming crude and emulsion through the entire cross section of the oil layer would increase retention times enormously. In some cases the results were 35 times better.
The conclusions of these studies proved that designs based on fluid dynamics could produce amazing results. When the design is optimized, the size of any wash tank or gun barrel tank will be much smaller and this will reduce the capital investment in the tank. Also, these new designs will produce better results regardless of the influences of outside of conditions such as summer-winter temperature swings. And surprisingly, researchers discovered that process capacity and separation efficiency increased even more when the inlet crude and emulsion stream is not washed through the water phase, but is instead introduced into the oil rich emulsion layer located just above the water phase. In fact, some studies showed that the inlet emulsion actually absorbs water from the water layer, increasing the BS&W concentration of the mix entering the bottom of the oil layer in a gun barrel.
These tests were conducted in the last great “oil boom” which started with the Arab oil embargo in the early 1970's and ended in 1985. While a great deal of the wisdom gained in those years was lost through “bust” cycle attrition of personnel and layoffs, the present invention used this knowledge in the present gun barrel design.
Too little effort had been given to the selection of oil process equipment in the past by oil companies, resulting in millions of dollars of unnecessary costs. Many oil operators simply pick up the phone and order a separator, a gun barrel or heater treater, a couple of oil tanks, a water tank, or whatever they ordered last time, even though the equipment they are ordering is not efficient at separating oil, gas, and solids from the produced oil. Very few equipment suppliers do any vessel sizing or application engineering for lease production equipment; most will simply ask the operators what they want and will fill the order because the suppliers presume that the operators knows what they want and need.
However, the new technologies we currently have available are completely different than the technologies available when our predecessors got away with a more casual approach to selecting new process equipment. Most of the equipment we use today was designed in a different time for different production conditions such as higher gas to oil ratios (GORs), much lower water cuts, higher pressures, less stable emulsions, and lower amounts of iron sulfide, sand, CO2, paraffin, etc. Not too far in the past, the norm was high oil volumes and very little water. Today, most operating conditions are just the opposite, with high water volumes and little oil. It shouldn't be too surprising that different conditions demand different equipment designs. It also shouldn't be too surprising that yesterday's process equipment designed for low water cut conditions doesn't fit well in today's higher water cut operations. Thus, use of older type separation equipment costs the oil industry a fortune each year.
The present invention is an improved gun barrel that is much more efficient, and this increase efficiency facilitates a smaller in size tank than traditional gun barrel tanks. The present invention was engineered with the knowledge that efficient and effective physical separation of oil from water in a gun barrel is a function of efficient fluid distribution and efficient fluid collection which then will result in longer retention time within a given size tank.
The required separation time in any application can be calculated using Stokes Law. However, the key to oil-water separation in facility design is to focus on distribution and collection, which result in real retention time improvements and far better overall separation.
The present invention focuses on these two areas. The invention employs a distributor trough designed to slow the inlet fluid velocity and provide uniform distribution of incoming flow into lateral distribution metering troughs located in the emulsion layer of the tank to provide uniform distribution of incoming fluids. The invention also employs a circular oil collection trough with a plurality of V-shaped notches provided on its edge that insure uniform collection of oil into the oil collection trough from around the entire circumference of the gun barrel tank. These two features make the present gun barrel extremely efficient at separating water from the oil.